1. Field of the Invention
The present invention relates to an image forming apparatus such a color laser printer and an image forming method.
2. Background Art
As a color laser printer, there is known a so-called four-cycle type color laser printer including four developing rollers provided according to the colors of cyan, magenta, yellow and black, a photosensitive body for forming electrostatic latent images developed by the developing rollers, an intermediate transfer body disposed opposite to the photosensitive body, and a transfer roller disposed opposite to the intermediate transfer body.
In the four-cycle type color laser printer, electrostatic latent images formed successively on the photosensitive body are developed successively by mono-color toners provided from the developing rollers provided according to the colors. In this manner, mono-color toner images are formed successively on the photosensitive body. The mono-color toner images are transferred successively onto the intermediate transfer body so as to be superposed on one another. In this manner, a multi-color toner image is formed on the intermediate transfer body. The multi-color toner image is transferred onto a sheet of paper by the transfer roller. Thus, a multi-color image is formed on the sheet of paper.
As the four-cycle type color laser printer, there is known a printer capable of performing simultaneous printing as follows. When, for example, the circumferential lengths of the photosensitive body and the intermediate transfer body are selected so that printing can be made on an A3-size sheet of paper, each of the photosensitive body and the intermediate transfer body are divided into two. Two A4-size pages' mono-color toner images are carried simultaneously on the photosensitive body and transferred successively onto the intermediate transfer body so as to be superposed on one another. As a result, two A4-size pages' multi-color toner images are carried simultaneously on the intermediate transfer body. The two A4-size pages' multi-color toner images are transferred continuously onto two A4-size sheets of paper by the transfer roller. Thus, simultaneous printing is completed.
Because of hardware characteristic of the color laser printer, such simultaneous printing, however, can be performed only in the condition that a variety of colors used for printing on the first sheet of paper are the same as a variety of colors used for printing on the second sheet of paper. That is, such simultaneous printing cannot be performed in the condition that a variety of colors used for printing on the first sheet of paper are different from a variety of colors used for printing on the second sheet of paper.
Therefore, for example, JP-A-11-115249 has proposed a printer having a multi-page simultaneous printing function such as a twin printing function for performing simultaneous printing on two monochrome or color pages.
In the process described in JP-A-11-115249, however, pages of a document to be printed are grouped into monochrome blocks and color blocks successively from the first page. Accordingly, unless the step of grouping all pages (up to the final page) into blocks is completed in one printing job, printing on the first page cannot be started. This is insufficient to attain shortening of the printing process.
In the process described in JP-A-11-115249, pages are only grouped into monochrome blocks and color blocks. There is no consideration of the case where a variety of colors used for color printing on one page are different from a variety of colors used for color printing on the other page, for example, the case where a front page has cyan, magenta and yellow whereas a rear page has magenta, yellow and black. In such a case, the printing process may become slow.